Tracer Arm and Its Graduation

CHAPTER II.

The Polar Planimeter.

Tracer Arm and Its Graduation.

In all better forms of Planimeters the Tracer Arm RS is graduated for almost its entire length into some scale of equal parts and with all its principle divisions and sub-divisions plainly marked. The principal use of this graduation is to allow of the recording of any position or point on the Tracer Arm and the certainty of any subsequent correct locating of that point when desired; it also furnishes a means of obtaining the exact distance measured along the Arm between the Tracer and the point in question.

The Scale or form of Graduation of the Tracer Arm varies with the different forms and makes of Instrument. In some Planimeters the graduation is in inches and decimals of an inch; in others the division is into half millimeters and fractions while in another form the Arm has been divided into fiftieths of an inch as being a sort of compromise between the Metric and English systems.

For reasons which will appear later a graduation into one-half millimeters as the principal divisions, with each such division subdivided into ten equal parts, making twentieths of a millimeter as the space between divisions lines, is perhaps preferable to any other form.

The beginning or Zero of the graduation may be at either end of the Tracer Arm, but placing the Zero at the tracer and having the numbers reading from left to right has many advantages not possessed by the other system of notation.

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